Abstract

Problem/Condition: Influenza epidemics occur nearly every year during the
winter months and are responsible for substantial morbidity and mortality in the
United States, including an average of approximately 114,000 hospitalizations and
20,000 deaths per year.

Reporting Period: This report summarizes U.S. influenza surveillance data
from October 1994 through May 1997, from both active and passive surveillance systems.

Description of System: During the period covered, CDC received weekly reports
from October through May from a) state and territorial epidemiologists on estimates of
local influenza activity, b) approximately 140 sentinel physicians on their total number
of patient visits and the number of cases of influenza-like illness (ILI), and
c) approximately 70 World Health Organization (WHO) collaborating laboratories in
the United States on weekly influenza virus isolations. WHO collaborating
laboratories also submitted influenza isolates to CDC for antigenic analysis. Throughout the
year, vital statistics offices in 121 cities reported deaths related to pneumonia and
influenza (P&I) weekly, providing a measure of the impact of influenza on mortality.

Results: During the 1994--95 influenza season, 25 state epidemiologists
reported regional or widespread activity at the peak of the season. Cases of ILI reported
by sentinel physicians exceeded baseline levels for 4 weeks, peaking at 5%.
Influenza A(H3N2) was the most frequently isolated influenza virus type/subtype. The
longest period of sustained excess mortality was 5 consecutive weeks, when the percentage
of deaths attributed to P&I exceeded the epidemic threshold, peaking at 7.6%.

During the 1995--96 season, 33 state epidemiologists reported regional
or widespread activity at the peak of the season. ILI cases exceeded baseline levels for
5 weeks, peaking at 7%. Influenza A(H1N1) viruses predominated, although
influenza A(H3N2) and influenza B viruses also were identified throughout the United States.
P&I mortality exceeded the epidemic threshold for 6 consecutive weeks, peaking at 8.2%.

The 1996--97 season was the most severe of the three seasons summarized in
this report. Thirty-nine state epidemiologists reported regional or widespread activity
at the peak of the season. ILI reports exceeded baseline levels for 5 consecutive
weeks,
peaking at 7%. The proportion of respiratory specimens positive for influenza peaked
at 34%, with influenza A(H3N2) viruses predominating. Influenza B viruses were
identified throughout the United States, but only one influenza A(H1N1) virus isolate was
reported overall. The proportion of deaths attributed to P&I exceeded the epidemic threshold
for 10 consecutive weeks, peaking at 9.1%.

Interpretation: Influenza A(H1N1), A(H3N2), and B viruses circulated during
1994--1997. Local surveillance data are important because of geographic and temporal differences
in the circulation of influenza types/subtypes.

Public Health Actions: CDC conducts active national surveillance annually
from October through May for influenza to detect the emergence and spread of
influenza virus variants and monitor the impact of influenza-related morbidity and
mortality. Surveillance data are provided weekly throughout the influenza season to
public health officials, WHO, and health-care providers and can be used to guide
prevention and control activities, vaccine strain selection, and patient care.

INTRODUCTION

Epidemics of influenza occur nearly every year during the winter months and
are responsible for substantial morbidity and mortality in the United States, including
an average of approximately 114,000 hospitalizations and 20,000 deaths per year
(1). Annual vaccination is recommended for groups at increased risk, including adults aged
>50 years, persons with underlying chronic health conditions (e.g., cardiovascular
disease, pulmonary disease, and certain metabolic conditions), and women in their
second or third trimester of pregnancy (1,2). During influenza epidemics,
hospitalization rates among older adults and persons with underlying chronic health problems
can increase twofold to fivefold over nonepidemic periods
(3). Influenza epidemics also are associated with increased mortality. From the 1972--73 season through the
1994--95 season, the average number of influenza-associated deaths was approximately
20,000 per year. During six of those seasons, >40,000 influenza-associated deaths
occurred (1,4). In recent years, >90% of influenza-associated deaths have occurred among
persons aged >65 years (5).

The Advisory Committee on Immunization Practices (ACIP) recommends
annual vaccination of persons at high risk for influenza-associated complications as the
most effective way to reduce the impact of influenza. ACIP also recommends annual
vaccination of persons in frequent contact with persons at high risk to reduce transmission
to those at high risk (1). Influenza vaccination is 70%--90% effective in preventing
influenza-like illness (ILI) in young, healthy adults when the vaccine antigens closely
match the circulating influenza virus strains
(6). Among nursing home residents aged
>65 years, influenza vaccine is 30%--40% effective in preventing illness, 50%--60% effective in
preventing serious complications and hospitalization, and 80% effective in preventing
death (7,8). In addition, vaccination can reduce the risk for outbreaks in nursing home
settings (9).

Influenza viruses undergo constant antigenic change. Because vaccine
effectiveness depends on antigenic similarity between vaccine strains and circulating
viruses, one or two of the three vaccine component strains typically are updated each
year. Both virologic surveillance, in which influenza viruses are isolated for antigenic
characterization, and disease surveillance are necessary to identify influenza virus
variants
and to determine their ability to spread and cause disease. This information is
needed for selection of the optimal influenza vaccine components each year.

Each year from October through May, weekly updates of summaries of U.S.
influenza surveillance data are available from the following sources:

Calling the CDC voice information system at 888-CDC-FACT (888-232-3228).

Calling the CDC fax information system at 888-CDC-FAXX (888-232-3299)
and asking for reports to be faxed to you (request document number 361100).

Influenza activity updates are published several times during each influenza
season in the Morbidity and Mortality Weekly
Report (MMWR [weekly]). This report
summarizes influenza activity for the three seasons from October 1994 through May 1997.

METHODS

The sources of influenza surveillance data used during the 1994--95, 1995--96,
and 1996--97 seasons were similar to those used in previous years. These sources are
listed below.

State and Territorial Epidemiologist Reports

The level of statewide or territory-wide influenza activity, as assessed by the state
or territorial epidemiologist, was reported to CDC weekly from October through May.
Levels were reported as either widespread (i.e., outbreaks of ILI* or culture-confirmed
influenza in counties having a combined population of
>50% of the state's population), regional (i.e., outbreaks of ILI or culture-confirmed influenza in counties having a
combined population of <50% of the state's population), sporadic (i.e., sporadically
occurring cases of ILI or culture-confirmed influenza, with no outbreaks detected), or no
activity. Methods of assessing activity levels vary from state to state.

Sentinel Physician Surveillance Network

Each week from October through May, approximately 140 volunteer
family-practice physicians reported the number of patients they saw each week, the number of
these patients who were seen for
ILI, and the number of these patients who were
hospitalized for ILI or related complications. ILI information was reported by age group.
Baseline levels of total patient visits for ILI ranged from 0% to 3%. Levels >3% usually
correlated with increased influenza activity. A subset of these physicians also submitted nasal
and throat swabs to a contract laboratory for virus isolation.

World Health Organization (WHO) Collaborating Laboratories

Each week from October through May, approximately 70 WHO collaborating
laboratories in the United States reported the number of specimens received for
respiratory virus testing and the number of positive isolations of influenza A(H1N1), A(H3N2),
A(not subtyped), or influenza B by age group (<1 years, 1--4 years, 5--24 years, 25--44
years,45--64 years, >65 years, or unknown). Most WHO collaborating laboratories were located
in state or local health departments, although some were in universities or
hospitals.These laboratories submitted a subset of the isolates for complete antigenic
characterization and antiviral resistance testing to the WHO Collaborating Center for Reference and
Research on Influenza at CDC. Although formal weekly reporting is discontinued
during summer months, WHO collaborating laboratories can report influenza viruses
isolated during the summer to CDC and submit these viruses for antigenic characterization.

121 Cites Mortality Reporting System

Each week throughout the year, the vital statistics offices in 121 cities reported
the number of death certificates filed and the number of death certificates in which a)
pneumonia was identified as the underlying cause of death or b) influenza was listed
anywhere on the certificate. These data were used to calculate the proportion of all
deaths attributed to pneumonia and influenza (P&I), as well as a P&I mortality curve. A
periodic regression model that incorporated a robust regression procedure was applied
to produce a seasonal baseline of P&I deaths and to calculate "excess" deaths above
the baseline. An increase of 1.645 standard deviations above the seasonal baseline of
P&I deaths was considered the epidemic threshold (i.e., the point at which the
observed proportion of deaths attributed to pneumonia or influenza was significantly higher
than would be expected at that time of year in the absence of influenza).

The data reported from the sentinel physician surveillance network and from
WHO collaborating laboratories are analyzed both nationally and by influenza
surveillance region (Figure 1). State and territorial reports include only state-based
surveillance, and data from the 121 cites mortality reporting system are only analyzed nationally.

RESULTS

1994--95 Season

State and Territorial Epidemiologist Reports

State and territorial epidemiologists first reported regional influenza activity for
the week ending December 3, 1994 (week 48) and widespread activity for the week
ending January 7, 1995 (week 1) (Figure 2). Influenza activity increased steadily during
January and February, peaked during the week ending March 11 (week 10) when 25 states
reported regional or widespread activity, then declined. Widespread activity was last
reported for the week ending April 15 (week 15), and regional influenza activity was
last reported for the week ending May 13 (week 19). Sporadic activity continued to be
reported through the week ending May 20 (week 20), the last week for which reports
were available.

Sentinel Physicians Surveillance Network

Visits to sentinel physicians for ILI exceeded baseline levels (0%--3%) for 4
consecutive weeks, from January 29 through February 25, 1995 (weeks 5--8) (Figure 3). The
percentage of visits for ILI peaked at 5% during the week ending February 18 (week 7).

Activity varied widely by influenza surveillance region. In the New England
region, the percentage of visits for ILI increased from 1% during the week ending January
7 (week 1) to 10% during the week ending January 14 (week 2), peaked at 17% during
the week ending February 18 (week 7), and remained above baseline levels through
the week ending April 8 (week 14). This peak level and duration of elevated activity
(13 consecutive weeks) was higher than in any other region. The Mountain region had
the second highest level of ILI activity, with patients visits for ILI peaking at 7%
during weeks 6, 8, and 9 and remaining above baseline levels for 8 consecutive weeks,
from January 8 through March 4 (weeks 2--9). In contrast, the percentage of patient visits
for ILI never exceeded baseline levels in the Mid-Atlantic, West North Central, East
South Central, and Pacific regions.

WHO Collaborating Laboratories and Vaccine Strain Selection

From October 2, 1994, through May 20, 1995, WHO collaborating laboratories
tested 39,657 respiratory specimens and identified 3,920 influenza isolates. Of these
3,920 isolates, 2,896 (74%) were influenza type A, and 1,024 (26%) were influenza B (Figure 4). Of the 1,940 influenza A isolates that were subtyped, 1,888 (97%) were identified
as influenza A(H3N2), and 52 (3%) were influenza A(H1N1). Reported influenza
isolates increased during December 1994 and January 1995, peaking from January 29
through February 11 (weeks 5 and 6). Influenza A(H1N1) viruses were first reported during
the week ending January 28 (week 4) and continued to be identified through the
week ending May 20 (week 20).

The proportion of influenza virus types and subtypes varied by region (Figure 5).
In the Mountain region, influenza B was the predominant virus and accounted for 98
(50%) of the 196 influenza viruses reported. Of the 70 influenza A viruses that were isolated
in
the Mountain region and subtyped, 18 (27%) were A(H1N1). Although the Mountain
region contributed only 196 (5%) of the total influenza isolates reported in the United
States, 18 of these were influenza A(H1N1) --- representing 35% of the national total of
H1N1 viruses.

Influenza B accounted for 38% of all influenza viruses isolated in the South
Atlantic region and was the predominant virus isolated in that region through the week
ending February 11 (week 6). In contrast, influenza B viruses accounted for 2% of isolates in
the East South Central region, 7% in the West North Central region, 10% in the East
North Central region, 12% in the New England region, and 15% in the West South
Central region.

Active influenza surveillance for the 1994--95 season ended May 20, 1995. After
this date, CDC's strain surveillance laboratory received additional influenza
A(H1N1), A(H3N2), and B isolates from specimens collected during late May and June. CDC
also received influenza A(H1N1) viruses collected during July and September,
influenza A(H3N2) viruses collected during August and September, and influenza B viruses
collected during August.

The 1994--95 influenza vaccine contained A/Shangdong/09/93(H3N2),
A/Texas/36/91(H1N1), and B/Panama/45/90 antigens (Table). Of the 311 influenza A(H3N2)
viruses antigenically characterized by CDC, 193 (62%) were similar to
A/Shangdong/09/93, whereas the remaining 118 (38%) were more closely related to
A/Johannesburg/33/94. A/Johannesburg/33/94-like viruses were isolated throughout the season, but the
proportion of H3N2 viruses similar to A/Johannesburg/33/94 increased as the
season progressed. Of the 58 influenza A(H1N1) viruses characterized, all but three were
antigenically related to A/Texas/36/91 or the antigenically similar virus A/Taiwan/01/86.
Forty-two (24%) of 173 influenza B viruses characterized were B/Panama/45/90-like
viruses, whereas the remaining 131 (76%) were similar to the antigenic variant
B/Beijing/184/93. The 1995--96 influenza vaccine was updated to include A/Johannesburg/33/94
(H3N2) and B/Beijing/184/93-like viruses while retaining A/Texas/36/91 as the H1N1 component.

121 Cites Mortality Reporting System

From October 2, 1994, through May 20, 1995, the percentage of deaths attributed
to P&I exceeded the epidemic threshold for 14 of the 33 weeks and peaked at 7.6% for
the week ending March 4 (week 9) (Figure 6). The longest duration in which this
percentage
was above the epidemic threshold was 5 consecutive weeks, from February 26
through April 1 (weeks 9--13).

Outbreak Surveillance

The first influenza outbreak reported to CDC during the 1994--95 season began in
late November and involved residents of a skilled nursing facility in Long Island, New
York (10). Influenza A(H3N2) viruses antigenically similar to A/Shangdong/09/93 --- the
influenza A(H3N2) component of the 1994--95 vaccine --- were isolated from eight
nasopharyngeal specimens. As the season progressed, influenza outbreaks were reported
among all age groups through May. Although most outbreaks were associated with
influenza A(H3N2) viruses, influenza A(H1N1) and influenza B viruses were also
associated with institutional outbreaks.

1995--96 Season

State and Territorial Epidemiologist Reports

During the 1995--96 influenza season, regional activity was reported by 0--2
states each week from October 1 through November 11, 1995 (weeks 40--45) (Figure 2).
The number of states reporting regional activity increased to five for the week ending
November 18 (week 46), and widespread activity was first reported the week
ending November 25 (week 47). Influenza activity peaked during the weeks ending January
6 (week 1) and January 13, 1996 (week 2), when 33 states reported regional or
widespread activity. Widespread activity was last reported for the week ending March
16 (week 11), and regional activity was last reported for the week ending April 20
(week 16).

The highest percentage of patient visits for ILI occurred in the East South
Central (15%), New England (12%), and Pacific (12%) regions. In contrast, the percentage
of patient visits for ILI in the Mid-Atlantic region peaked at 4%. The number of
weeks above baseline levels varied by region from 13 weeks in the New England region to
1 week in the Mid-Atlantic region.

WHO Collaborating Laboratories and Vaccine Strain Selection

During the 1995--96 influenza season, the number of isolates reported from
WHO collaborating laboratories rose sharply during November and peaked during the
week ending December 23, 1995 (week 51), when 26% of respiratory specimens tested
were positive for influenza (Figure 4). From October 1, 1995, through May 18, 1996, a total
of 4,740 influenza isolates were reported to CDC. Of this number, 4,018 (85%) were
influenza type A, and 722 (15%) were type B. Although influenza A viruses
predominated overall, influenza B isolates increased at the end of the season and were isolated
more frequently than influenza A during March, April, and May. Of the 2,571 influenza
A isolates subtyped, 1,507 (59%) were influenza A(H1N1). The last report of
influenza A(H1N1) isolates occurred during the week ending March 23 (week 12). Isolation
of influenza A(H3N2) viruses continued through April and May. Active influenza
surveillance for the 1995--96 season ended on May 18, but WHO collaborating
laboratories continued to report influenza B isolates from specimens collected each month
through July. Influenza A(H3N2) viruses were isolated from specimens collected each
month from May through September.

The temporal pattern of influenza virus isolations was similar throughout the
country, with virus isolation peaking in all regions from week 50 through week 52.
However, the proportion of different virus types/subtypes varied among regions (Figure 5).
Influenza A was the predominant virus type isolated in all nine regions. However, at the
subtype level, influenza A(H1N1) viruses were predominant in six regions (Mid-Atlantic,
East North Central, West North Central, South Atlantic, East South Central, and West
South Central), whereas influenza A(H3N2) viruses were predominant in the New
England, Mountain, and Pacific regions. The highest percentages of influenza B isolates were
reported in the East North Central (25%) and West North Central (23%) regions. In
contrast,
3% of the isolates identified in the New England region and 5% of those in the
Pacific region were influenza type B.

The 1995-96 influenza vaccine contained A/Texas/36/91(H1N1),
A/Johannesburg/33/94(H3N2), and B/Beijing/184/93-like antigens (Table). For the B/Beijing/184/93-like
virus, U.S. vaccine manufacturers used the antigenically equivalent virus B/Harbin/07/94
because of its growth properties. Of the 219 influenza A(H3N2) viruses characterized,
155 were similar to A/Johannesburg/33/94. Sixty-three isolates were more closely
related to A/Wuhan/359/95, and the proportion of A/Wuhan/359/95-like isolates increased
over time. All of the 211 influenza A(H1N1) isolates antigenically characterized by CDC
were similar to the reference strains A/Texas/36/91 or A/Taiwan/01/86. All influenza B
viruses were similar to B/Beijing/184/93. A/Texas/36/91(H1N1) and B/Beijing/184/93-like
strains were retained for the 1996--97 vaccine. The H3N2 component of the
1996--97 vaccine
was updated to an A/Wuhan/359/95-like (H3N2) virus.

121 Cites Mortality Reporting System

For 1995--96, the percentage of deaths attributed to P&I exceeded the epidemic
threshold for 6 consecutive weeks, from December 24, 1995 (week 52), through February
3, 1996 (week 5), and peaked at 8.2% during the week ending January 20, 1996 (week
3) (Figure 6). The proportion of deaths attributed to P&I returned to baseline levels
during the week ending February 10 (week 6) and did not exceed the epidemic threshold
again for the remainder of the season.

Outbreak Surveillance

Although most outbreaks reported to CDC during the 1995--96 influenza season
were among school-aged children, some were among adults aged
>65 years residing in nursing homes. In 1996, influenza A(H3N2) viruses were associated with a) a nursing
home outbreak in Washington from late May through June, b) a nursing home outbreak
in Hawaii during July, c) increased influenza activity at a military base in Hawaii during
July and August, and d) an outbreak among students at a Wisconsin university that began
in mid-September (11,12).

1996--97

State and Territorial Epidemiologist Reports

During the 1996--97 influenza season, regional activity was first reported for the
week ending October 19, 1996 (week 42), and continued to be reported by one or two
states each week through the week ending November 16 (week 46) (Figure 2). For the
week ending November 23 (week 47), the number of states reporting regional activity
increased to seven, and widespread influenza activity was reported for the first time.
Influenza activity continued to increase during December and peaked during the week ending
January 4, 1997 (week 1), when 39 states reported regional or widespread activity.
Activity declined from late January through March. The last report of widespread activity was
for the week ending March 22 (week 12), and regional activity was last reported for the
week ending April 12 (week 15).

The highest percentage of patients visits for ILI occurred in the New England
and Pacific regions, which peaked at 21% and 16%, respectively. These regions also
had more weeks above baseline levels --- 13 weeks for the New England region and 14
consecutive weeks in the Pacific region. In contrast, the peak percentage of patient visits
for ILI in the East South Central region (2%) was below baseline levels.

WHO Collaborating Laboratories and Vaccine Strain Selection

During the 1996--97 influenza season, the number of isolates reported by WHO
collaborating laboratories peaked during the weeks ending December 21 (week 51)
and December 28, 1996 (week 52), when 33% and 34%, respectively, of specimens
tested for respiratory viruses were positive for influenza (Figure 4). Of the 6,509
influenza isolates reported to CDC from September 29, 1996, through May 17, 1997, a total
of 5,056 (78%) were influenza type A, and 1,452 (22%) were type B. Influenza type A
viruses were reported for all weeks during the 1996--97 season, with the highest number
reported during the week ending December 21 (week 51). Of the 2,274 influenza A
isolates subtyped, 2,273 were influenza A(H3N2) and one was influenza A(H1N1). Influenza
type B viruses were reported during 28 of 33 weeks and peaked during the week ending
March 1 (week 9). Influenza B viruses were more frequently isolated than type A from the
week ending February 15 (week 7) through the end of the season.

The temporal pattern of influenza virus isolation was similar throughout the
country, with peaks occurring in all nine regions from week 50 through week 1. In all regions,
a peak of influenza B activity followed the peak of influenza A activity. Influenza A was
the predominant virus type isolated, ranging from 62% in the Pacific region to 89% in
the West South Central region.

The 1996--97 influenza vaccine contained A/Wuhan/359/95-like (H3N2),
A/Texas/36/91(H1N1), and B/Beijing/184/93-like antigens (Table). For the A/Wuhan/359/95-like
strain, U.S. vaccine manufacturers used the antigenically equivalent strain
A/Nanchang/933/95(H3N2) because of its growth properties. For the same reason, manufacturers
used B/Harbin/07/94 for the B/Beijing/184/93-like strain. All 319 influenza A(H3N2)
viruses antigenically characterized by CDC were similar to A/Wuhan/359/95. Of the 158
influenza B viruses characterized antigenically, all were similar to B/Beijing/184/93.
No influenza A(H1N1) viruses from the United States were submitted during the
1996--97 season. Based on the antigenic characterization of H1N1 viruses from other
countries, the H1N1 component of the 1997--98 influenza vaccine was updated to an
A/Bayern/ 07/95-like (H1N1) virus. Because of its growth properties and suitability for vaccine
production, U.S. manufacturers used A/Johannesburg/82/96 (H1N1). The H3N2 and B
components did not change for the 1997--98 season.

121 Cites Mortality Reporting System

During the 1996--97 influenza season, the percentage of deaths attributed to P&I
exceeded the epidemic threshold for 13 of 33 weeks (Figure 6). This percentage
exceeded the epidemic threshold for 10 consecutive weeks, from December 8, 1996 (week
50), through February 15, 1997 (week 7), and peaked at 9.1% during the week ending
January
25 (week 4).

Outbreak Surveillance

Outbreaks associated with influenza A(H3N2) viruses were reported during the
summer of 1996 (11), until the beginning of the 1996--97 season, and throughout the
fall and winter (12). During the 1996--97 influenza season, most reported outbreaks
occurred among nursing home residents aged
>65 years. However, all age groups were
affected, with outbreaks among students also frequently reported.

DISCUSSION

Influenza type A viruses predominated during each of the three influenza
seasons from October 1994 through May 1997. Influenza A(H3N2) was the prevalent
subtype during the 1994--95 and 1996--97 seasons, and A(H1N1) was more prevalent during
the 1995--96 season. During the 1995--96 season, the predominating influenza A
subtype varied by regions. Influenza B viruses accounted for 15%--26% of all influenza
viruses isolated during each of these three seasons, and the proportion of influenza B
viruses increased toward the end of the 1995--96 and 1996--97 seasons.

The differences in the temporal and geographic distribution of influenza
viruses across the United States illustrate the importance of timely, ongoing influenza
surveillance at the local, state, and regional levels. This information helps health-care
providers determine the optimal treatment for patients with ILI throughout the United States.

Although widespread influenza activity typically occurs in the United States
during the winter months, sporadic cases and outbreaks can occur at any time of the
year. Influenza viruses were isolated and reported to CDC each month from November
1994 through May 1997, and outbreaks of influenza A were reported throughout the
summer of 1996. Because influenza infection can occur at any time of the year, physicians
should include influenza in the differential diagnosis of febrile respiratory illness during
summer months. Several rapid diagnostic tests for influenza are available and can be
performed in <30 minutes in a physician's office. One test type detects only influenza
type A viruses, but other tests can detect both influenza A and B viruses without
differentiating between the virus types. These tests are useful for the rapid identification of
influenza, but should be used along with viral culture, particularly during institutional
outbreaks and for cases that occur during nonpeak months.

Since their identification in 1968--69, influenza A(H3N2) viruses typically have
been associated with increases in P&I mortality, particularly among older adults.
Influenza A(H3N2) viruses predominated during eight of the 11 influenza seasons during
1972--1995 in which there were >20,000 excess deaths attributed to P&I. The
1994--95
influenza season was unusual in that influenza A(H3N2) viruses predominated, but P&I
mortality as detected by the 121 cites mortality reporting system was low. The percentage of
P&I deaths exceeded the epidemic threshold by a small margin for 5 consecutive weeks
and peaked at 7.6%. Influenza morbidity rates reported by sentinel physicians also were
low, with the percentage of patient visits for ILI peaking at 5% nationally and never
exceeding baseline levels in four of the nine regions. During the 1996--97 season,
influenza-related mortality followed a pattern more typical of years predominated by H3N2 viruses,
with the proportion of deaths attributed to P&I exceeding the epidemic threshold for 10
consecutive weeks, peaking at 9.1%.

Before the 1995--96 influenza season, influenza type A(H1N1) viruses had not
circulated widely in the United States since the 1988--89 season, when they represented
almost 50% of influenza virus isolates reported by WHO collaborating laboratories in
the United States (13). The occurrence of a high proportion of reported outbreaks
among school-aged children during the 1995--96 season is consistent with patterns seen
during previous influenza seasons when type A(H1N1) viruses have predominated
(14). Influenza A(H1N1) viruses circulated from 1918 through 1957, then disappeared for 20
years. The influenza A(H1N1) virus that reappeared in 1977 was antigenically and
genetically similar to strains isolated in 1950 and 1951. Since their reappearance in 1977,
influenza A(H1N1) viruses have had less impact on persons born during or before the
mid-1950s than on those born after that time, probably because of immunity developed during
the 1940s and 1950s (14).

After the 1996--97 influenza season, surveillance methods were changed.
Sentinel physician surveillance for influenza is now conducted in collaboration with state
health departments. As a result, the number of participating and regularly reporting
physicians has increased to approximately 400. Virologic information is now received
from laboratories participating in the National Respiratory and Enteric Virus
Surveillance System, as well as from WHO collaborating laboratories, increasing the number
of laboratories reporting each week to approximately 120.

During each influenza season, activity updates of preliminary data are
published several times in the MMWR [weekly]. Data summaries for entire seasons are
published periodically in CDC Surveillance
Summaries.

References

CDC. Prevention and control of influenza: recommendations of the Advisory Committee
on Immunization Practices (ACIP). MMWR 2000;49(No. RR-3).

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